The invention relates generally to an on-demand type ink jet print head, and in particular to an ink jet print head in which ink is stored in an ink tank and drops of liquid ink are jetted through print head nozzles by piezoelectric elements onto a recording medium, such as paper.
On-demand type ink jet print heads can be classified into three categories. A first type of print head is commonly referred to as a bubble jet print head. This type includes a heater located at the ink jet nozzle and ejects ink drops by vaporizing a localized portion of ink and utilizing the expansion pressure generated by the vaporized ink to force ink from the print head.
A second type of print head includes a piezoelectric element provided in a vessel defining an ink reservoir. Ink drops are ejected by force generated by localized variations in pressure in the ink reservoir caused by the deformation of the piezoelectric element.
A third type of print head includes an assembly of reed-like pieces including a piezoelectric portion in an ink reservoir having an ink drop ejection orifice. The reed pieces are positioned opposite the ink drop ejection orifice with the liquid ink present between the reed piece and the orifice are ejected due to pressure generated by piezoelectric deformation of the reed piece.
The third type of ink jet print head is described in Japanese Patent Publication No. 60-8953. This shows a print head including a housing defining an ink tank that has a plurality of nozzle openings in a wall of the housing. A reed piece having a piezoelectric portion is aligned with each nozzle opening and each reed piece is actuated by a print signal to jet drops of ink from the print head. As the print head operates, the reed piece is deformed away from the nozzle opening by an electric print signal. When the applied signal ends, the reed piece rapidly returns towards the nozzle opening due to its inherent resilient characteristics. As the reed piece returns, it causes a drop of ink to fly from the nozzle opening.
The piezoelectric element utilized in the third type of print head is shaped like a reed and is supported in cantilever form to provide a large amount of displacement. This type of print head is advantageous because ink can be ejected with high efficiency, and the ejection operation is not significantly adversely influenced by gas, dust, etc. that are inadvertently included in the ink. The operating reliability of this type of print head is very high.
However, this type of print head also has disadvantages. The assembly of reed pieces is conventionally formed by machining a piezoelectric plate into the form of a comb with the reed pieces are the teeth of the comb. Thus, all of the reed pieces are both mechanically and electrically connected on their root end to the same base portion of the piezoelectric plate. Accordingly, adjacent reed pieces influence each other both mechanically and electrically. This causes mutual interference and results in the vibration mode of each reed piece becoming unstable.
This type of print head also has manufacturing disadvantages. A comb shaped reed piece assembly is conventionally formed by making cut lines midway in a raw piezoelectric plate and leaving a root portion unmachined. Such a configuration leads to low yields and high breakage. It is also difficult to attach an electric signal wire to an electrode on the piezoelectric plate if the electrode is formed on the free vibration side of the reed piece to avoid problems associated with electrical mutual interference. Thus, conventional ink jet print heads are not fully satisfactory and have inadequacies due to these shortcomings.
Accordingly, it is desirable to provide an improved on-demand type ink jet print head which avoids the shortcomings of these prior art.